1. Field of the Invention
The present invention relates to a display device where display is effected by applying a driving signal to a pixel electrode via a switching element. More specifically, the present invention relates to an active matrix display device where high-density display is effected by arranging pixel electrodes in a matrix.
2. Description of the Related Art
Display devices such as a liquid crystal display device, an EL (electroluminescence) display device, and a plasma display device are conventionally operated by methods where a display pattern is formed on a screen by selecting desired pixels among a plurality of pixels arranged in a matrix. As one of such methods, an active matrix driving method is known.
In the active matrix driving method, pixel electrodes arranged in a matrix are independently driven by controlling switching elements connected to the respective pixel electrodes. Such an active matrix driving method realizes high contrast display and thus has been practically used for displays of liquid crystal TV sets, wordprocessors, computer terminals, and the like. TFTs (thin film transistors), MIM (metal-insulator-metal) elements, MOS transistors, diodes, and the like are generally used as switching elements for selectively driving the pixel electrodes. A voltage applied across each pixel electrode and a counter electrode disposed to face the pixel electrode is switched by use of the corresponding switching element, so as to optically modulate a display medium such as liquid crystal material, EL material, and plasma light emitting material contained between the electrodes. This optical modulation of the display medium is visually recognized as a display pattern.
The pixel electrodes are often formed together with source bus lines or gate bus lines in the same layer. In such cases, the pixel electrodes are arranged not to be in contact with the source bus lines and the gate bus lines.
Japanese Laid-Open Patent Publication No. 61156025 proposes a display device where pixel electrodes and bus lines are formed in different layers with an insulating film formed on the bus lines and under the pixel electrodes. With this configuration, since a larger area of each pixel electrode can be secured, the aperture ratio of the resultant display device improves.
FIG. 19 is a plan view of one pixel of the above-described display device. FIG. 20 is a sectional view taken along line A--A of FIG. 19. A TFT is used as the switching element.
As shown in FIG. 19, gate bus lines 1111a and 1111b and source bus lines 1112a and 1112b run along a pixel portion of the display device. Each of the gate bus lines 1111a and 1111b includes a plurality of projections 1111 (FIG. 20) serving as gate electrodes of the TFTs. As shown in FIG. 20, the gate bus lines 1111a and 1111b having the projections 1111 are formed on a substrate 1110, and an insulating film 1130 is formed thereon. On the insulating film 1130, a semiconductor layer 1131 is provided above each projection 1111 so as to serve as a channel region of the TFT. The source bus lines 1112a and 1112b (FIG. 19) each of which includes a plurality of projections 1114 serving as source electrodes of the TFTs are formed on the insulating film 1130 so that the projections 1114 partially overlap the semiconductor layers 1131 with other semiconductor layers 1132 therebetween. Also, drain electrodes 1115 of the TFTs are formed on the insulating film 1130 by, for example, patterning the same layer as that constituting the source bus lines 1112a and 1112b and the projections 1114 serving as the source electrodes.
As shown in FIG. 20, an insulating film 1133 is formed over the entire substrate 1110 so as to cover the gate bus lines 1111a and 1111b, the source bus lines 1112a and 1112b and the TFTs each including the projection 1111 as the gate electrode, the projection 1114 as the source electrode, the drain electrode 1115 and the semiconductor layers 1131 and 1132. A contact hole 1116 is formed through the insulating film 1133, so that a pixel electrode 1140 can be electrically connected with the drain electrode 1115 via the contact hole 1116. The pixel electrode 1140 is therefore in a separate layer from the gate bus lines 1111a and 1111b and the source bus lines 1112a and 1112b. Accordingly, the pixel electrode 1140 can be formed to overlap the gate bus lines 1111a and 1111b and the source bus lines 1112a and 1112b, thereby improving the aperture ratio of the device, as shown in FIG. 19.
In the fabrication of a display device for high-density display, electrical leakage may occur between the pixel electrode 1140 and the source bus line 1112a or 1112b or between the source bus lines 1112a and 1112b due to insufficient cleaning of a substrate, attachment of dust to the substrate, and the like at the fabrication of the substrate on which switching elements are to be formed. Such leakage may bring about a point defect or a line defect of the resultant display device. High technology is required to fabricate a large-scale display panel including some millions of pixels without bringing about such defects.
Referring to FIGS. 21 and 22, cases of leakage between the adjacent source bus lines 1112a and 1112b (hereinbelow, such leakage is called SS leakage) for some reason will be described. This leakage results in two line defects on the display. If a display panel with these defects is used to complete a display device, the resultant displayed device is found to be defective. This lowers production yield.
In the case where such leakage occurs due to a conductive pattern piece 1121 left unremoved as shown in FIG. 21, a drain electrode 1115 located between the source bus lines 1112a and 1112b is also electrically connected with these source bus lines. As a result, the pixel electrode 1140 can no more hold a required potential. This is exhibited as a point defect. Such a point defect can be located and the leak portion can be cut by laser trimming or the like at a cut portion 1122 shown in FIG. 21, for example, at the stage where the display panel including the defective pixel has been completed. In this way, the SS leakage can be eliminated. Thus, by locating and eliminating SS leakage in a display panel before the display panel is incorporated in a display device, the display panel which once had SS leakage can be used for a product.
In the case where SS leakage occurs due to a conductive pattern piece 1161 left unremoved in the middle of the pixel as shown in FIG. 22, no point defect is exhibited. The leakage occurs only between the source bus lines 1112a and 1112b. Only line defects are therefore recognized when a screen is lighted and displays an image. Repair of this type of leakage is considerably difficult because no point defect as a mark for leakage repair is exhibited. Though it is not impossible to locate a leak portion by scanning along the source bus line 1112a or 1112b with a microscope, it requires an enormous amount of time. Such scanning is therefore impracticable on a practical production line. This type of leakage may occur more significantly when pixel electrodes and source bus lines are formed in different layers with an insulating film therebetween. Especially, this leakage may occur when a transparent conductive film such as an ITO film is used as source bus lines.